This invention relates to a silver halide photographic material, more particularly to a photographic material that contains a compound capable of working as a novel nucleating agent.
Nucleating agents are known to be used as contrast increasing agents in silver halide photographic materials in photochemical processes. They are also known be useful as foggants in direct positive-acting silver halide photographic materials.
The photochemical process includes the step of converting the change of density in a continuous tone image to a set of halftone dots having areas proportional to the image density, and silver halide photographic materials having contrasty photographic characteristics are generally used in this step. In order to impart contrasty characteristics to image, hydrazine compounds are contained as "contrast increasing agents" in silver halide photographic materials and, in addition, silver halide grains that help these compounds exhibit the inherent contrasty characteristics effectively are used or other appropriate photographic additives are used in combination with those hydrazine compounds in such a way as to obtain desired photographic materials. For details of these techniques, see JP-A-56-106244 (the term "JP-A-" as used herein means an "unexamined published Japanese patent application"), U.S. Pat. No. 4,686,167 and European Pat. No. 333,435. The silver halide photographic materials are indeed stable light-sensitive materials and can produce high-contrast photographic image even if they are processed with developing solutions adapted for rapid processing.
However, if these photographic materials are used in the step of converting a continuous tone image to a halftone image, sandlike fog or pepper fog which are generally referred to as "black dots" can occur in halftone dots, leading to impaired dot quality. In an attempt at solving this problem, various stabilizers or restrainers having hetero atoms have been added but this has not always proved to be a complete solution. Under these circumstances, a light-sensitive material that uses an effective contrast increasing agent free from that problem is desired.
A known method of forming a positive image using a direct positive-acting silver halide photographic material is to perform imagewise exposure using a yet to be fogged internal latent image forming silver halide emulsion and to conduct surface development in the presence of a foggant, thereby producing a positive image. Various techniques of this approach have been known and are described in such prior patents as U.S. Pat. Nos. 2,592,250, 2,456,957, 2,497,875, 2,588,982, British Patent No. 1,151,363, JP-B-43-29405 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-47-9434, 47-9677, 47-32813, 47-32814, 48-9727, 48-9717, U.S. Pat. Nos. 3,761,266, 3,496,577, and JP-A-50-8524 and 50-38525.
Hydrazine compounds are known as useful foggants. Exemplary hydrazine compounds that have been used as foggants include the hydrazine compounds described in U.S. Pat. Nos. 2,563,785 and 2,588,982, the naphthylhydrazinesulfinic acid described in U.S. Pat. No. 2,064,700, and the sulfomethylhydrazines described in British Patent No. 1,403,018. JP-B-41-17184 teaches that a positive color image can be produced using hydrazide or hydrazone compounds. However, the use of these hydrazine compounds has the problem that the induction period (the time required for image to become visible upon development) is longer than in the case of ordinary development with latent-image silver and, hence, the development with these compounds is considerably retarded.
Another problem with the prior art is that when it is applied to multi-layered color photographic materials, unevenness is likely to occur in photographic characteristics between layers or the finally obtained maximum density is low.
Further, development is conventionally performed at pHs higher then 12 in order to maintain desired fogging action, thereby assuring satisfactory results. However, this approach is by no means advisable since it either causes significantly accelerated deterioration of developing agents or impairs the film properties of the processed photographic material. Therefore, it is also desired to develop direct positive silver halide photographic material that uses an advantageous foggant free from the aforementioned problems.